OK, then it likely has a center differential, which will help things. I knew of availability of AWD 4Runners, but don’t know what years they were offered. And I don’t know what ratio split front/rear they normally run (most AWD systems can vary torque split depending on conditions).

Basically what you have is two tires of approximately the same mileage and wear; a slightly used tire and a new one. Given that you maintained them by regularly rotating them you should be OK. Differences in circumference is slight to begin with, but is real and it is there. The center and rear differentials should negate the slightly different turning speeds and circumferences of your various tires on the pavement without undue wear or damage. But over the time before you swap out all four tires it might have an effect in increased wear, even if it is a bit more metal shavings in your differential oils. And your tires will also wear slightly faster. But catastropic failure? No. Unless something’s ready to go already. Given it is a Toyota, I doubt it. They are tough trucks.

It is major differences - large differences in tire size - that can cause damage on the road now or later. Over the decades I’ve seen plenty of vehicles with major differences in tire size front/rear and wince when I think of the circumstances when they pull back on the transfer case lever.